1. Field of the Invention
The present invention relates to a semiconductor laser array. It can be used in numerous fields such as optical telecommunications, optical pumping, treatment of materials, etc.
2. Prior Art
The increase in the power of semiconductor lasers cannot be obtained merely by increasing the active surface, because instabilities of modes and emitter filaments which are poorly controlled occur. It is therefore necessary to define elementary emitters formed by active areas having small lateral dimensions. The power increase is then obtained by multiplying said elementary emitters.
A considerable beam quality improvement is obtained by ensuring a coherence of the optical phase between the emitters by coupling them together. The simplest coupling is obtained by evanescent wave (i.e. by covering the evanescent parts of the modes of two adjacent elementary emitters). This type of coupling favours the oscillation of two adjacent emitters in phase opposition, which leads to a multilobe radiation pattern. In the case of multistrip lasers, where the lightwave propagates parallel to the semiconductor surface, various means have been proposed for ensuring an inphase coupling, which produces an emission in a single lobe, perpendicular to the exit face (generally obtained by cleaving). This method is e.g. described in the article by J. Katz et al "Diffraction coupled phase locked semiconductor laser array" published in Appl. Phys. Lett., 42, (7), Apr. 1, 1983, pp. 554-556 and L. J. Mawst et al entitled "CW high-power diffraction-limited-beam operation from resonant optical waveguide arrays of diode lasers" published in Appl. Phys. Lett., 58 (1), Jan. 7, 1991, pp. 22-24.
The recent development of vertical cavity lasers emitting by the surface makes it possible to envisage matrixes of emitter points instead of an emitting line, as well as an emission perpendicular to the surface of the semiconductor chip and not on the edge, without involving technical complications. Such matrixes are in particular described in the article by M. Orenstein et al entitled "Two-dimensional phase-locked arrays of vertical-cavity semiconductor lasers by mirror reflectivity modulation" published in Appl. Phys. Lett., 58 (8), Feb. 25, 1991, pp. 804-806.
In general, a matrix of lasers of this type comprises a conductive substrate and, on said substrate, a lower mirror, an active layer (optionally surrounded by confinement layers) and an upper mirror formed by a matrix of reflecting areas (e.g. a matrix of 4.times.4 areas). The coupling between the different vertical elements is obtained by diffraction at the edges of the reflecting areas. The complete structure behaves like a plurality of lasers (16 in the case of a 4.times.4 matrix) slightly coupled together.
Although such structures are interesting in certain respects, they still suffer from the disadvantage of leading to a multilobe beam, which is therefore only slightly directional. This disadvantage is due to the fact that the coupling produced leads to a phase opposition between adjacent elementary lasers, which causes an anti-symmetric radiation pattern.